Method and apparatus for the preparation of transparent alumina ceramics by microwave sintering

ABSTRACT

An apparatus ( 10 ) for the development of transparent alumina ceramics using microwave energy at the frequency between 0.915 and 2.45 GHz inclusive in hydrogen atmosphere at ambient pressure comprises an enclosed, insulated chamber ( 14 ) to retain a workpiece ( 12 ) for the application of microwave energy. The chamber comprises a TE 103  single mode or a multimode microwave cavity into which is mounted a quartz tube ( 18 ). An insulation material ( 20 ), transparent to microwave energy, is positioned within the quartz tube. A port ( 28 ) for the introduction of hydrogen penetrates the cavity so that the microwave sintering of the workpiece is performed in an ultra-pure hydrogen atmosphere. The workpiece is preferably mounted on a refractory ceramic such as alumina tube for the microwave sintering process. A method, preferably using the apparatus, develops transparent alumina ceramics and single crystal sapphire.

This application claims the benefit of U.S. Provisional Application No.60/222,845 filed Aug. 4, 2000, titled “Sintering of polycrystallinealumina to translucency using microwave sintering, for lightingapplications and gems”.

FIELD OF THE INVENTION

The present invention relates generally to the field of alumina ceramicsand, more particularly, to a method and an apparatus for makingtransparent alumina ceramics using microwave sintering.

BACKGROUND OF THE INVENTION

Transparency is a valuable optical property in certain opticalmaterials. In polycrystalline materials, a number of factors influencethe degree of transparency of the material including grain size,density, crystal structure, porosity, and the grain boundary phase.Glasses are optically isotropic, monolithic (i.e. have no grainboundaries), and therefore possess excellent transparency. So can cubicceramics. All non-cubic ceramics are anisotropic and polycrystalline.The grain boundaries in the ceramic strongly scatter light. However, ifthe grain size is smaller than the wavelength of the light (0.4-0.7 μm),then light can transmit through the ceramic. If the grain size is largerthan the wavelength of light, by minimizing grain boundaries andimpurity content, the material can be made transparent or translucent.

To achieve transparency in a ceramic, one must control grain growth,eliminate porosity, and achieve a fully dense material. Conventionalmethods of fabricating fully dense and reasonably transparent ceramicsinvolve high temperatures, lengthy sintering times, and various complexprocessing steps, which not only make the processing of transparentceramics uneconomical but often the desired properties are not achieved.

Transparent alumina (Al₂O₃) ceramics can be prepared either in singlecrystal (have generally termed, sapphire) or polycrystalline forms.Sapphire is used in many industrial and military applications, such asoptical windows for lasers, spectrometers, armor parts, and IR-domes forinfrared missile guidance systems. Also, synthetic sapphire gemstoneshave become a popular jewel material. Other common alumina ceramicsinclude, for example, MgAl₂O₄ (spinel) and (Na,Ca) Al₁₂O₁₉ (β-alumina).Polycrystalline transparent alumina for optical applications was firstmade in the early 1960s as described in U.S. Pat. No. 3,026,210, issuedto Coble. In Coble, polycrystalline alumina bodies having the desiredoptical properties were made by preparing a mixture of high purityfinely divided alumina powder with {fraction (1/16)} to about ½ weightpercent of finely divided magnesia (MgO). The method comprisedcompacting the mixture of finely divided alumina and magnesia, andfiring the compact for predetermined periods of time at a temperaturenot lower than about 1700° C. in a vacuum or a hydrogen environment. Theresultant polycrystalline transparent alumina has become a key elementin high-pressure sodium vapor lamps and other optical instrumentsmanufactured throughout the world. The cost to manufacturepolycrystalline transparent alumina is much lower than that of sapphireand it is easier to produce in large size products.

As described in Coble, and in techniques which have become well known inthe art, polycrystalline transparent alumina is made via powderprocessing using high purity and fine particle sized alumina powder withthe addition of a small amount of MgO, and sintering to pore-free state.Sintering is essential in obtaining high transparency material. In theconventional sintering process, extremely high sintering temperatures(up to 1900° C.) and long soaking times (several hours) under highvacuum or pure hydrogen atmosphere are applied in the fabrication oftransparent alumina products to achieve the highest density and minimumporosity.

Microwave sintering is a new technique for ceramic materials processingwhich differs fundamentally from current conventional processes justdescribed. In microwave processing, samples positioned in a microwavefield absorb microwave energy and convert it into heat directlyproviding volumetric heating. As a result, a microwave process providesseveral advantages, such as more rapid and uniform heating, shorterprocessing time, fine microstructure, enhanced energy efficiency, andimproved materials properties and product performance. Enhanceddensification behaviors are also provided when microwave processes areused due to a reduction in the activation energy for sintering, whichleads to a lower sintering temperature and shorter sintering timecompared to conventional sintering processes.

Microwave sintering of alumina material is known. The early work onmicrowave sintering of alumina ceramics was performed in 1975 andreported by W. H. Sutton. In that work, over 1360 kg of productionshapes of alumina castables were successfully fired using microwaveenergy. J. Katz et al. reported the successful sintering of relativelylarge samples (about 1 kg) of high purity, undoped Al₂O₃ to about 93%theoretical density (T.D.) in a 2.45 GHz multimode cavity. M. Janney andH Kimrey found that the alumina sample could achieve a density up to 98%T.D. at 1100° C. when microwave sintered at 28 GHz, and they suggestedthat compared to conventional sintering processing, the sinteringactivation energy is much lower when microwave radiation is applied,which leads to higher densification rate at lower temperatures in amicrowave field. One of the inventors of the present invention, J.Cheng, and his co-workers investigated the densification kinetics ofalumina, and found that the diffusion coefficient during microwavesintering was three times higher than that in the conventional sinteringat the same temperature. None of these efforts resulted in fully dense,and therefore optically transparent alumina articles.

In U.S. Pat. No. 5,451,553, Scott et al. describe a solid state processfor the conversion of polycrystalline alumina to sapphire material. Inthe described process, a polycrystalline material containing less than100 ppm by weight of magnesia was reheated to temperatures above 1100°C., but below the melting point of alumina, in a high purity hydrogenatmosphere for 300 hours, or at 1880° C. for 3-9 hours. While effective,the described process is too time consuming and expensive for the largescale production of sintered alumina with adequate transparency.

The first attempt to prepare transparent ceramic samples by microwavesintering processing was conducted by Y. Fang et al. in 1995-1996. Usingspecially synthesized precursor powder, transparent hydroxyapatite andtranslucent mullite samples were made using a microwave sinteringtechnique.

In Japanese Laid-Open Patent Application No. 7-187760, laid open Jul.25, 1995, a method for manufacturing artificial sintered gemstone isdescribed. A synthetic-gemstone starting material powder, obtained byadding chromium oxide, titanium oxide, and/or other oxides to an aluminapowder and a magnesia powder is molded, and the resulted molding is thensintered by being heated at 1300 to 1800° C. with microwaves at 2.45 to200 GHz in a reduced atmospheric pressure (vacuum) of 100 to 0.01 Pa toproduce a synthetic gemstone. While this reference provides no detailsof the apparatus, certain characteristics can be discerned from themethod described. At the reduced pressure of the vacuum, the specifiedfrequency range is called for in order to attain adequate heating,probably by creating a plasma. Further, the reference provides nopre-heating of the molded material, and neither describes nor suggeststhe use of a hydrogen atmosphere.

Thus, there remains a need for an efficient, cost effective method and astructure for microwave sintering of polycrystalline alumina ceramics toa transparent body. The present invention is directed to this need.

SUMMARY OF THE INVENTION

The present invention provides a method and an apparatus for theformation of transparent ceramic bodies from polycrystalline alumina(Al₂O₃). The apparatus comprises an enclosed, insulated chamber toretain a workpiece for the application of microwave energy. The chambercomprises a single or multimode microwave cavity into which is mounted aquartz tube. An insulation material, transparent to microwave energy, ispositioned within the quartz tube. A port for the introduction ofhydrogen penetrates the cavity so that the microwave sintering of theworkpiece is performed in an ultra-pure hydrogen atmosphere. Theworkpiece is preferably mounted on a refractory tube such as alumina forthe microwave sintering process.

The apparatus just described is used to carry out the microwavesintering method of the invention. A starting Al₂O₃ powder with magnesiaof 0.05% by weight is used to form a workpiece of the desired size andshape, such as for example by molding. The sample or workpiece ispreferably formed by uniaxial press at 300 MPa pressure and calcined at1100° C. for two hours in a conventional furnace for debindering. Theworkpiece is then placed inside the microwave chamber previouslydescribed and sintered, for example, at 0.915 to 2.45 GHz in a singlemode cavity or a multi-mode cavity at power levels of 1.5 kW to 6 kW.Ultrahigh purity hydrogen can be applied as a sintering atmosphere forsintering at ambient pressure. Typically, the heating rate is 150° C.per minute in the single mode cavity and 100° C. per minute in themulti-mode cavity. High density and translucency are obtained bymicrowave sintering at 1700° C. for only 10 minutes, but sintering up to30 minutes provides a more highly transparent alumina product. Thismethod may be used to produce an alumina ceramic of Al₂O₃ compositionand a corundum crystal structure. Further, this method may be used toproduce an alumina based ceramic which has the β-Al₂O₃ ormagneto-plumbite crystal structure or MgO.(1-3)Al₂O₃ and the structureof spinel.

In a further aspect of the present invention, the transparent aluminaproduct obtained from the process just described may then be subjectedto another microwave sintering step in order to develop a single crystalproduct. The sane apparatus previously described is used for thisfurther processing step. This further sintering step produces a corundum(or sapphire) product.

These and other features of the invention will be apparent to those ofskill in the art from a review of the following detailed descriptionalong with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation section view of the apparatus of the presentinvention.

FIG. 2 is a section view of a sintered alumina ceramic body.

FIG. 3 is a microscopic view of a specimen produced by rapid microwavesintering without the use of magnesia as a sintering aid.

FIGS. 4a, 4 b, and 4 c depict microstructures of alumina using magnesiaas a sintering aid for various lengths of time.

FIG. 5 is an overhead view of a specimen sintered in accordance withthis invention illustrating the transparency of the specimen.

FIG. 6 is a plot of transmittance versus frequency for a microwavesintered sample at various soak times.

FIGS. 7a and 7 b depict X-ray diffraction patterns for Al₂O₃ specimentreated with a single step microwave sintering (a), and the specimenwith an additional heating step (post-sintering) to convert into singlecrystal alumina (b).

FIG. 8 is a photomicrograph illustrating the microstructure developmentof Al₂O₃ single crystal area resulting from post-sintering treatment bymicrowave.

FIG. 9 is an overhead view of Al₂O₃ specimen sintered in accordance withthis invention illustrating the transparency of the specimen as a resultof post-sintering treatment by microwave.

FIG. 10 is a plot of transmittance versus frequency for a microwavesintered Al₂O₃ sample, illustrating the improvement in transmittancefollowing post-sintering treatment by microwave.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 illustrates a microwave sintering system 10 constructed inaccordance with the present invention. A compacted sample or workpiece12 mounts within a chamber 14. The chamber 14 is formed by a TE₁₀₃single mode cavity 16 with a quartz tube 18 securely mounted therein.Mounted within the chamber 14 is a tube of microwave transparentinsulation 20, which effectively serves to maintain a more uniformheating of the sample or workpiece 12 during the sintering process to bedescribed below.

A window port 22 attached to the chamber 14, preferably at the top ofthe chamber, to monitor the temperature of the workpiece is shown inFIG. 1. The pyrometer 24 is used to monitor and control the sinteringtemperature within the chamber 14 of the sample 12. A tube 26,preferably formed of Al₂O₃ to prevent contamination of the Al₂O₃workpiece, penetrates into the chamber 14 to serve as a pedestal for thesupport of the workpiece 12. A port 28 also penetrates into the chamber14 for the supply of hydrogen to the chamber. A diffuser 29 may also beprovided to reduce the velocity of hydrogen flow around the sample 12since such flow rate may cause a thermal gradient within the sample dueto a cooling effect of the hydrogen. In operation, hydrogen purges thechamber to provide a pure hydrogen environment for the microwavesintering of the workpiece, and the hydrogen is maintained atatmospheric pressure.

A source 30, preferably a 1.5 kW microwave generator, provides microwaveenergy through the quartz tube 18 and the insulation 20 into the chamber14 to sinter the workpiece 12. A shield and reflector 32 closes off thecavity 16 to eliminate microwave leakage from the system 10 and toreflect microwave energy back into the chamber 14 for peak efficiency ofthe system. In operation, the source 30 provides microwave energy,preferably at 0.915 to 2.45 GHz to effectively sinter the workpiece 12in a hydrogen environment.

The workpiece is formed, for example, by the dry pressing uniaxially ofa high purity (99.99%) commercial alumina powder, such as Baikalox CR10,Baikoski International, NC, USA. The properties of Baikalox aluminapowders are listed in Table 1 and Table 2. As shown in Table 1, theprimary particle size, measured in microns, is 0.15 and as shown inTable 2, the starting material has less than 150 parts per millionimpurities, and as used herein, the starting material is substantiallypure.

TABLE 1 The properties of Baikalox alumina powder. Product code CR10Specific Surface Area (BET-m²/g) 8 ± 1 Major crystal phase alpha Majorphase content (%) 90 Primary particle size (microns) 0.15

TABLE 2 The chemical analysis of Baikalox alumina powder (maximumimpurity/ppm by weight) Na K Si Fe Ca Other cations 20 50 40 10 5 <5

To form the sample, the alumina powder is blended in acetone with 0.05%by weight of MgO (in form of Mg(NO₃)₂.5H₂0) and a binder using aluminamortar. Green samples are then prepared by dry pressing uniaxially intopellets, or in other desired shape, followed by cold isostatic press(CIP) at a pressure of 280 MPa, for example. The green densities of thecompacts are preferably around 52-54%. The compacted pellets arepre-heated at 1100° C. for 2 hours using a conventional resistancefurnace to burn out the binder. Rather than pellets, workpieces of adesired shape and configuration may also be formed, such as, forexample, discs which may be used as optical components or tubes whichmay be used in high-pressure sodium vapor lamps or other opticalinstruments.

The pre-heated, compacted workpieces are then placed into the chamber14. Microwave sintering is carried out using the source 30, a TE₁₀₃single mode microwave applicator, coupled with a 1.5 kW microwavegenerator operating at 0.915 to 2.45 GHz for small samples (less than0.5 inch diameter), or a multimode microwave applicator with a 6 kWmicrowave power source for large samples (up to one inch diameter).Ultrahigh purity hydrogen under 1 atmosphere pressure is introduced intothe chamber through the port 28 providing the sintering atmosphere.Sintering is preferably performed in a temperature range of 1700 to1900° C. with a soaking time of 10 to 60 minutes. Magnesia as asintering aid has been found to be effective in the rage of 0.025 to0.25% by weight, and preferably at 0.05% by weight.

To digress briefly, FIG. 2 shows a section view of a sinteredpolycrystalline Al₂O₃ ceramic body, showing the effects of the variousfactors on the optical properties of the body. The transmissivity of thebody is measured as a ratio of the transmitted light, I, over theincident light, I₀. Various factors affect this ratio. First, some ofthe incident light is scattered from the surface of the body by diffusescattering. Further, grain size relative to the frequency of theincident light can affect the travel of light through the body. Otherfactors affecting the transmission of light through the body includegrain boundaries, second phase layers (impurities), precipitation(foreign bodies), and pores (gas pockets between grain boundaries). Thefunction of the sintering process of this invention is to reduce theeffects of these factors on transmissivity.

Using MgO as a sintering aid is a key factor in making transparent Al₂O₃ceramic samples. The mechanistic role of MgO in the sintering of Al₂O₃has been studied for several decades. MgO plays two important roles. Onerole is to increase surface diffusion, which keeps the pores mobileenough to remain in contact with the grain boundary until the pores areannihilated by solid state diffusion. The other role is to decreasegrain boundary mobility that reduces the pore boundary tendency to avoidpore entrapment within grains. However, the extra addition of MgOresults in more second phase, MgAl₂O₄ spinel, formed at the boundary asshown in FIG. 2, which worsens the samples' transparency.

To demonstrate the importance of the MgO as a sintering aid, some pureAl₂O₃ samples with no addition of MgO were prepared using microwavesintering at a very high heating rate (550-600° C./min) to 1850° C. anddwelled at that temperature for 5 minutes under H₂ atmosphere. Thedemonstration was intended to investigate if a very high heating rateand short sintering time by microwave radiation could provide a fullydense and fine-grained Al₂O₃ body. The density of the sample was 3.83g/cm³, about 96.5% of theoretical density. It was found that thesintered body has enormous grain size (a few hundred microns), with lotsof small pores trapped within the grains. A microscopic view of theresult is shown in FIG. 3. The sample shown in FIG. 3 is only partiallytranslucent with some visible cracks. This demonstration teaches that asintering aid such as MgO is indispensable, and that extremely greatgrain growth rate can be obtained during microwave sintering.

FIGS. 4a, 4 b, and 4 c illustrate the beneficial effects of using 0.05%MgO by weight (in the form of Mg(NO₃)₂.5H₂O) as sintering aid for Al₂O₃samples. These figures show the microstructures of the samples microwavesintered at 1750° C. with dwelling times of 15 minutes, 30 minutes, and45 minutes, respectively, at heating rate of 100° C./minute. As it isshown, these samples exhibited very neat grain boundary structure anduniform grain growth with no porosity. All samples had a density of 3.97g/cm³(˜100% of theoretical density), but the average grain sizeincreased from 20 microns to 40 microns while the sintering timeincreased incrementally from 15 minutes to 45 minutes. The appearance ofthe transparent Al₂O₃ sample that was microwave sintered at 1750° C. for45 minutes is shown in FIG. 5. A specimen 50 is positioned over ameasuring rule 52, in order to illustrate the transparency of thespecimen.

The transmittance measurements of the microwave sintered Al₂O₃ samplesare shown in FIG. 6. Plots 60, 62, and 64 depict transmittance inpercent for the samples of FIGS. 4a, 4 b, and 4 c, respectively. Notethat the transmittance is slightly higher for samples that are microwavesintered for longer periods of time. For comparison, the transmittanceof single crystal Al₂O₃ (sapphire) is about 80% at all wavelengthswithin the frequency range of FIG. 6.

As previously shown and described with respect to FIG. 2, compared tosingle crystals, a sintered polycrystalline Al₂O₃ ceramic body possessesmuch more complicated microstructures that consist of grains, grainboundaries, second phases and pores, which greatly influence theiroptical properties. The transmissivity (I/I₀) is dependent not only onthe crystal characteristics of the material, but on the grain size andboundary structure:

I/I ₀=(1−R)²exp(−μx).  (1)

where I is the intensity of transmitted light exiting the sample body,I₀ is the intensity of incident light, R is the reflectivity, μ is theabsorption coefficient, and x is the thickness of a sample body. Theabsorption coefficient μ can be given as

μ=α+S _(im) +S _(op).  (2)

where α is the absorption term characteristic of electron transition,S_(im) is the scattering due to structural inhomogeneities such as poresand second phase, and S_(op) is the scattering due to opticalanisotropy. Since Al₂O₃ has a hexagonal rather than cubic crystalstructure, and the light gets scattered at interfaces such as grainboundaries where refractive indices are discontinuous, and as aconsequence, transmitted light becomes diffuse. In this case, S_(op) isalways an issue. To increase the transmissivity of a sinteredpolycrystalline Al₂O₃ body, it is most important to reduce S_(im). Thatis, it is important to reduce porosity, reduce the grain boundaryvolume, and densify as much as possible by optimizing the sinteringprocess. Finally, the best approach to improve the transparency is toremove boundaries to eliminate the S_(im), and S_(op) in equation (2),that means to convert the polycrystalline to single crystal structure.

Another feature of the present invention comprises a two step microwavesintering process. In that aspect of the invention, a workpiece of Al₂O₃with 0.05% by weight MgO doping, is microwave sintered to transparencyand then subjected to post-sintering treatment in a microwave field atelevated temperature, for example 1850-1880° C., under ultrahigh purityhydrogen atmosphere for a period of time sufficient to convert it intosingle crystal alumina. The same apparatus depicted in FIG. 1 is alsoused for this step. In a practical example, a 0.375 inch diameter Al₂O₃(as-sintered transparent sample by microwave sintering at 1750° C. for30 minutes) supported by the high purity Al₂O₃ tube 26 was placed in asingle mode microwave cavity to apply microwave post-sinteringtreatment. It was observed that there were some temperature differencesbetween the center and periphery area of the Al₂O₃ disk sample. Forexample, when the center area reached 1880° C., the temperature aroundthe peripheral area was 1850° C. This may be due to hydrogen gas flowingby the workpiece, so to reduce this effect, the diffuser 29 may be used.

FIGS. 7a and 7 b show the X-ray diffraction (XRD) patterns of theas-sintered Al₂O₃ sample (i.e. using a single step of microwavesintering) and the sample subjected to post-sintering step in microwave,respectively. The as-sintered sample's XRD pattern exactly tallied withthe standard Al₂O₃ (corundum) powder XRD pattern (FIG. 7a). The samesample was microwave post-sintered at 1850° C. for 2 hours. Its x-raypattern shows only {116} crystal plane as the dominant orientation, andthere were still some small peaks corresponding to the polycrystallinephase (FIG. 7b).

FIG. 8 reveals the changes in the microstructures of the post-sinteredAl₂O₃ sample that was microwave heated at 1880° C. (temperature in thecenter area of the disk sample) for 30 minutes. It is obviouslydemonstrated that the conversion of the polycrystalline phase to singlecrystal alumina during the microwave post-sintering processing has takenplace. The microstructure showed that peripheral area of the sampleremained in polycrystalline structure with average grain size of 30-40microns (the top area in FIG. 8); the center part of the post-heatedsample had no grain boundary and converted to a single crystal.

The microwave post-sintered sample is shown in FIG. 9, and itstransmittance behavior is shown in FIG. 10. In comparison with FIG. 4, a20% increase of the transmittance was achieved by microwavepost-sintering step.

In the microwave field, the densification and grain growth of Al₂O₃ceramic was enhanced in a great extent. The sample, microwave sinteredat 1750° C. for only 15 minutes, showed fall densification and goodtransparency. The addition of small amount of MgO as sintering aid isindispensable to achieve pore-free structure. The microwavepost-sintering step provides a much faster processing method for thesolid-state conversion of polycrystalline Al₂O₃ sample into singlecrystal sapphire.

It is also possible to make colored transparent alumina by the additionof small amounts of transition element. Such transition elements in theform of dopants include Cr₂O₃, V₂O₅, NiO, CuO, and TiO₂, which may beadded to the alumina-magnesia mixture to obtain various colored gems ofpolycrystalline alumina.

The principles, preferred embodiment, and mode of operation of thepresent invention have been described in the foregoing specification.This invention is not to be construed as limited to the particular formsdisclosed, since these are regarded as illustrative rather thanrestrictive. Moreover, variations and changes may be made by thoseskilled in the art without departing from the spirit of the invention.

We claim:
 1. A method of sintering alumina based ceramics, comprisingthe steps of: a. forming a selected alumina ceramic powder into aworkpiece; b. applying microwave energy to the formed workpiece in ahydrogen atmosphere at atmospheric pressure until the workpiece istransparent.
 2. The method of the claim 1, wherein the alumina ceramicis of Al₂O₃ composition and has a corundum crystal structure.
 3. Themethod of claim 1, wherein the alumina ceramic has the Al₂O₃ ormagneto-plumbite crystal structure.
 4. The method of claim 1, whereinthe alumina-ceramic has the composition of MgO.(1-3) Al₂O₃ and structureof spinel.
 5. The method of claim 1, wherein the microwave energy isapplied at between about 0.915 GHz and about 2.45 GHz, inclusive.
 6. Themethod of claim 1, wherein the composition and structure of thetransparent workpiece are those of sapphire and further comprising thestep of applying microwave energy a second time until the workpieceforms a single crystal.
 7. The method of claim 6, wherein the secondmicrowave energy is different than the first microwave energy.
 8. Themethod of claim 6, wherein the second microwave energy is the same asthe first the microwave energy.
 9. A system for sintering an aluminaceramic workpiece comprising: a. a chamber adapted to retain an aluminaceramic workpiece; b. an insulation layer lining at least a portion ofthe chamber; c. a temperature measuring instrument adapted to measurethe temperature of a workpiece within the chamber; d. a port into thechamber for the supply of hydrogen to the chamber; and e. a source ofmicrowave energy adapted to subject the workpiece to microwave energy ina hydrogen atmosphere at atmospheric pressure until the workpiece istransparent.
 10. The system of claim 9, further comprising a mountadapted to support the workpiece.
 11. The system of claim 9, wherein themount is made of refractory ceramic.
 12. The system of claim 9, whereinthe source of microwave energy provides microwave energy to theworkpiece at 0.915 GHz to 2.45 GHz inclusive.